This is a multicenter collaborative study to determine the sensitivity, specificity, positive predictive value, and negative predictive value of contrast enhanced spiral computed tomography (spiral CT) for the diagnosis of acute pulmonary embolism (PE). Pulmonary embolism is common, yet frequently undiagnosed and fatal. Ventilation/perfusion lung scans are the most commonly used test. However, they are nondiagnostic in 72 percent of patients with suspected PE, and in 57 percent of patients with proven PE. Pulmonary angiography is the currently definitive diagnostic test, but there is hesitancy to use it because of morbidity, discomfort, cost, and lack of availability in community hospitals. Contrast enhance spiral CT is a nearly noninvasive test that offers the possibility of a definitive diagnosis of PE by showing the outline of the thrombus in a pulmonary artery. However, its utility in the diagnosis of PE is unknown. Even though spiral CT has not been adequately validated, physicians at many hospitals now use it as a definitive and only diagnostic test for PE, or as a definitive test in patients in whom the ventilation/perfusion lung scan is nondiagnostic. This could lead to overtreatment or undertreatment, both of which have serious potential complications. The role of spiral CT in the diagnosis of PE requires an accurate evaluation which will be made in this investigation by comparison with pulmonary angiography. The strengths of this proposal are: 1) the importance of the problem of PE in terms of lives lost from underdiagnosis, and major bleeding from overdiagnosis 2) the potential applicability of a new technology for the diagnosis of PE that will have widespread availability and 3) the investigator team. Noninvasive leg tests, particularly venous ultrasonography, permit a strategy of management by treatment on the basis of venous thromboembolic disease (PE or deep venous thrombosis), ventilation/perfusion lung scan, or compression ultrasound of the lower extremities. Pulmonary embolism will be diagnosed on the basis of a high probability ventilation/perfusion lung scan in patients with no prior PE, pulmonary angiography, or compression ultrasound of the lower extremities in patients with no prior deep venous thrombosis. Deep venous thrombosis detected by venous ultrasound will serve as a surrogate for the diagnosis of PE. Pulmonary embolism will be excluded by pulmonary angiography, or a nearly normal or normal ventilation/perfusion lung scna with no adverse outcome (i.e. PE or DVT) on follow-up of untreated patients. Among patients with a low probability ventilation/perfusion lung scan, PE will be excluded by 2 negative venous compression ultrasound studies at 1 week intervals followed by 6 months of follow-up with no treatment and at 1 week intervals, followed by 6 months of follow-up with no treatment and no recurrent PE or DVT. The protocol parallels standard diagnostic strategy, without subjecting the patients to risk entirely for the acquisition of data.